False positive for Down's with amnio?

My sister just learned from her preliminary amniocentesis that her baby girl is positive for Down's Syndrome. The final report is due next week. Both my sister and I are RN's. We both know that this means in all probability her baby does have Down's. But, I am hopeful that there is someone out there who has gone through the AFP, Level 2 ultrasound, and amnio test with results that read "positive" when the baby was actually fine? Just looking for hope.